Attempts mean : the SDCCH channel is given to the MS (the BSC have successfully allocated the subchannel in the cell), in other words the reference of the SDCCH timeslot & subchannel is sent to the MS in the Immediate Assignment.
Success : the MS has replied to the Imm Asg, and therefore the MS has succesfully landed on its SDCCH subchannel.
if you have 100 attempts and 90 success :
it means 10 attempts did not becme success. 10 MS have not succeeded to land onto their SDCCH. Probably radio problems : the SDCCH is interfered, or the MS is too far away (bad coverage)